Preferred method for checking threads

[Larry1958]

We are having some issues with Q.C. dept. (imagine that). Day to day we machine fine threads on lens barrels. We normally use a couple methods for checking them, home made plug, and ring gages, 3 wire method, and fitting to mating parts. We have recently started a Q.C. dept. that checks our parts. Issue is they are using pitch micrometers to check with and not getting same reading as our methods. Trying to tell them that 3 wire method and there pitch mics are different by as much as .002" (o.d. threads and always on the + side). They say they will reject parts if they are not checking good using pitch mics. Bottom line is they fit mating parts and check good with wire method. Normally we machine anywhere from 48tpi, to 64tpi on dias. from .75" - 1.5" on Ultem2200 and 6061 alum. I need some ammunition from fellow machinist and Q.C. personal to determine best practices. My worries are that we could be making out of tolerance parts and Q.C. will accept them, customer may have certified ring gages and may have no-go conditions. Thanks in advance and sorry to stir this pot up.

[Guyinthedesert]

I would go by the three wire method over a pitch mic any day.

[K2csq7]

Anyone who has been around threads will tell you 3 wires are the way to go, not to mention you have been making good parts for how long using your methods??????? <if it ain't broke.....

I have many times had customers supply gages for "special" applications, a phone call request for a gage can't hurt. Pitch mics also have a tendency to read big because there is only 1 point of contact during calibration, if the tip is worn slightly (rolled over) it will contact 2 places in the threads before the "tip" that isn't true anymore gets to the root.

Thread gages are #1 IMO.

[jeff]

Absolute BEST way is to have GO/NO-GO ring gages special made (not home made gages).

This may not be the cheapest route to go, but IMO it's the best way to check threads.

 

If I don't have gages, then the 3 wire method is my preference.

[Jaz]

Ask how the pitch mics were calibrated and how they are verified for accuracy. The only way to prove pitch mics are good is to measure a known (certified) pitch diameter with the same pitch threads that needs to be checked. Maybe buy a couple of plug gages and demonstrate the accuracy of your method vs. theirs.

[CADCAM3D5AXIS]

I go with the ring gauges and certs to back up the cal on them, and 3 wire .also if QC wants to be up to date you can have them buy a Nice Laser Calibration tool that scans the threads and gives you a print out of any and all dims.

HTH

[Aeroguy]

Persoanlly I have found most QC guys to be some of the laziest and most unknowledgable people in the industry, with the rare exception of a couple here and there. Bottom line is, if they are properly trained QC people they will use either 3 wire method or an outside made thread guage. Thread mics are for rough checking only.

[Dan@home]

Get the (calibrated) ring gadges. To many vairibles can and will acure using other methods.

[Larry1958]

Thanks guys. Great responses. Keep them coming. I will relay all information with management and strongly suggest purchasing gages for all of our production parts.

[Hardmill]

Get the (calibrated) ring gadges. To many vairibles can and will acure using other methods.

 

 

^^^^^^ This is spot on.

They shouldnt need any other proof.

Have you asked your customer their preffered method of inspection ??

They are the one who should drive this subject.

 

 

 

 

 

 

 

 

 

 

 

PEACE

[Larry1958]

Just to be fair. Are there any Q.C. guys here that agree with checking these threads using Pitch micrometers?

[taperlength]

I like to look at outside threads on the optical comparator to check the thread form. Certified gage is the best way in the shop, next would be wires. Mating part would be my last option. One time they gave me a mating part that was oversize.

[JAMMAN]

I wore out a certified ID thread gage one time over the course of 3 years and had to recall about 100 orbital forming heads after the assembly tech locked one on a spindle and destroyed both spindle and head getting them apart. The heads were steel but only 11L17 so they weren't that tough.

 

I too believe the gage is the way to go but have it re-certified every so often depending on production, would have paid for itself in my case.

[Newbeeee™]

Ring gauge is always the preferred method.

We have in the past had to release parts when we didn't have a ring gauge, but we then used the thread mic, shadowgraph, and then made a nut as well, just to ensure the thing ran on as there can easily be a burr that you can't see.

Functional gauging is the way to go every time.

[Guest]

I always try to find out how they will be inspected at incoming inspection, that is the way I want to be be checking them.

 

Ring gauges are good but I have seen situations where the end of the thread will gauge good but further in there's an issue, so a back up and secondary check is always good.

 

For traceability, a gauge made in-house, unless it has been certified by an outside source, should NEVER be used to check a customers part.

 

 

Can you imagine a customer rejecting a part, auditing your QC and you pull out the home made gauge?

 

Were it me, I'd be pulling my work out as as I walked out the door.

[Bruce Caulley]

I always like thread mics for simplicity, BUT, if the thread was functional or tight tolerance in any other way I would check the mic on a calibrated male gauge of known PD rather than just zeroing it. This way when I did compare using wires I very rarely had a discrepancy.

 

Having said that a QC dept should be going for accuracy and repeatability over ease of use. If their mics aren't matching wires then they have a problem.

 

Bruce

[M. Anderson]

1. Go - No-Go gage rings - or thread plugs if thread milling(certified, not in house set or made)

2. thread mic's to check for taper (mine are ususaly dead nuts to the tolerance of the ring gage's)

3. 3 wire for odd size's only

 

Shop and QC need to use the same method - WHAT EVER METHOD IS USED!!!!!!!!!!

 

Ring gage's ALL need to come from same supplier....

Thread Mic's all need to be same brand (MIT is what I use).....

 

 

HTH

[Guest CNC Apps Guy 1]

Thread Ring or Thread gages are my favorite. They are traceable to NIST which is usually the last word. They do requiire a bit of discipline when using (i.e. keeping track of # of turns/uses for calibration purposes) and proper lubrication before and proper cleaning and storage after use. After that, three wire. There's a pretty far drop in accuracy with anything after that. Mics are always suceptible to "feel" which is tough. All that being said, find out what your customer uses and go with that method. That way you're at least on the same page.

 

JM2C

[Müřlıń®]

Your inspection process must be on par with the customers QC.

 

I agree with go/nogo as best.

 

Everyones feel is different and must be taken out of the loop for total precision.

 

If you use wires, use an indicator mic, not a standard thread mic.

[Guest CNC Apps Guy 1]

If you use wires, use an indicator mic, not a standard thread mic.

 

[Larry1958]

Just checked a certified plug gage with pitch micrometers. Size is .92810-48 - pitch should read .9146 +.0002 Got an .0008-.0010 error on the + side. Not a good way to go IMO.

[JerryBenoit]

There are several good ways to check threads each has there own place. I trend to feel it is ok to use ring gages but never trust them completely. Reason being my last employer have to swallow his pride and rework over a 1000 peices of 718 inco because the ring gages wore out (Q.C. gave the green light on when the gages neede to be swaped they were a little off). Wire meathod is good difficult to use by some but they are accurate. Pitch mics also are good, however it does not matter what method you use. If you only use and that one method is wrong do to calabration, a wore gage, or miss use. You will be reworking parts! Using a mating part is not good because is it to the high or low of it accecpted tolerance. After that episode we adopted policy to check every feature with a double method. Both have to match if not it was up to Q.C. to decide how the parts are to be gaged. Again looking at the history if the double check method did not verify the accuracy. Which one of you want to explain to your employer or customers why the threads are not to spec? Each method is good each should repeat if not do some gage R&R.

 

Jerry

[crazy^millman]

Okay here is my thoughts on this. I personally bought anvil Thread Mics for sizes from 1" to 4" and yeas I had well over $1500 in them. I cut threads for well over 20 years on types of different equipment. I could hold tenths on thread P.D. with no problem. I knew how to use them and how to read them. With threads at 48 tpi and 64 tpi I would not recommend thread mics, just to the small size of the thread to get an effective and accurate measurement. Go-NoGO would be my only choice from a quality standpoint and from a manufacturing standpoint. The use a part to check has bit me and unless I got it on the P.O. part supplied is required to check threads to then I always made them to the standard. That way if anyone ever came back to question the quality of a part I had any hand in I could always fall back on the standard. We made some Class 3 Threads for Spirit Aero one time. The Id on the Stainless 15-5 Part was Class 3 and the Od threads on the Mating part were class 3 as well with no special P.D. called on either part. I raised all types of red flags and no none would listen. These were for the some of the 1st 747-8's coming off the line at Boeing and the delivery was cut close. We go to put the parts together and even making sure the male parts were made to low side of the P.D and the Id parts were made to big side of the P.D. they still galled going together. We get screamed at and yelled at for the quality of the work. I made sure source bought them off at the component stage. I made sure they noted the P.D. of each one before assembly. I then went back to the records and some engineers had to eat some major crow. We got a premium on those replacement parts. Everyone of the parts were checked using thread mics and everyone of the P.D.s on those things were within tenths of each other. The Go fit and the no-go did not. Most time people do not pay attention to the fact you have to set the mic to s standard if it is an anvil thread mic. They replace the anvils without every checking it to the supplied standard. If done the right way for the right thread the right tool with do what it supposed to do, but with any tool if not calibrated and not used the right way then nothing is going to do it at all.

 

Just my thoughts on this one for what it is worth.

[JerryBenoit]

Ron,

 

Good point, so often we assume the customer is correct in the requirements we tend to forget that cutting threads is more than just getting the P.D. to size. We also need to have the Major, Minor, and Form fall into tolerance as well. If you look at any insert manufacture they produce class 2 type cresting threading inserts, but if you need to cut a class 3 you will need to cut the crest with another tool, or special order inserts for the thread you desire to cut.

Now when cutting any thread I always look at the P.D. then check the thread form. I make sure that when you cut the P.D. to its size limits does the minor fall within, if the insert is cresting does the major fall in. Reason being is I have seen the customer requirements wrong that when you get the P.D. size the minor or major would be out.

One case comes to mind was a customer gave us a special ACME thread that gave a tolerance on the P.D., major, and minor on that thread of (+/- .01). I thought peice of cake however, after looking at the root radius they required. "You cannot get there from here!" The minor would be way below the tolerance they gave. Not only that, but we would have cut a spiral ring around the bearing journal next to the threads.

When inspect threads you need to look at the P.D. and thread form. If you fail to look at the whole characteristic, someone will eat crow. Better them then you.

 

Jerry

[J Coulston]

I don't have anything to add that hasn't already been covered, other than a styrofoam peanut works well for holding small wires for measuring PD with the 3 wire method.